With gas seals of, for example, the type disclosed in EP 0,499,370 and EP 0,578,377, the disclosures of which are incorporated herein by reference thereto, groove areas are provided in one of a pair of opposing sealing faces whereby, upon rotation of one of the sealing faces relative to the other, a cushion of product gas is built up between the sealing faces to lubricate and create a seal between the sealing faces. When such seals are not rotating, they rely on engagement between the sealing faces to provide a static seal. Even though the grooved sealing face will normally have a continuous circumferential dam formation which will engage the other sealing face, gas seals of this type are subject to leakage under static conditions. When used in high pressure applications, for example on gas compressors, the expansion of the gas as it leaks across the gas seal will cause cooling of the gas which may even lead to liquefaction of the gas. The presence of liquefied gas in the gas seal is detrimental to the efficient operation of the gas seal under dynamic conditions. In order to address this problem auxiliary compressors have hitherto been used to pass warm gas through the gas seal. A further problem with leakage under static conditions is that the leaking gas must be vented or flared to atmosphere, with the consequent adverse affect on the environment.